RFC: Support Claude Code VS Code extension sessions

[perry-the-agent]

What do you want to do?

Use txcript with sessions created by the Claude Code VS Code extension. txcript supports Claude Code sessions from the CLI, but extension-created sessions should also be discoverable and portable through the same list, view, convert, and export workflows.

Example session workflow

1. Start or continue a Claude Code session in the VS Code extension.
2. List that session with txcript.
3. View it or convert/export it to another supported harness without losing messages, tool calls, timestamps, or workspace metadata that the extension persists.

Proposed behavior

• Identify the extension's session storage locations and schema on supported platforms.
• Reuse the existing Claude Code adapter where the formats overlap; add extension-specific discovery/parsing where they differ.
• Make extension sessions available through the existing txcript CLI and library APIs, with a clear source identifier if needed.
• Preserve all supported conversation content and metadata.
• Handle duplicate sessions when the CLI and extension expose the same underlying conversation.
• Add sanitized fixtures, discovery/parser tests, and user-facing documentation.

Acceptance criteria

• Extension-created sessions can be discovered, listed, and viewed with txcript.
• They can be converted/exported through the existing workflows.
• Discovery works across supported operating systems and VS Code profiles/workspaces.
• Duplicate CLI/extension representations are not returned as separate sessions.